Black Mold was a strange, dusky substance Paul Church discovered while inside the Farmed Hive. It was destructive to Xenomorph biology. It has no effect on Human biology.
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For his cooperative behavior, Paul Church was allowed by the sickly Xenomorphs of the farmed Hive to wander about. In these wanderings, he discovers the mold, which is destructive to Xenomorph biology and their food sources. He cultivated and used this mold as a secret weapon against his captors.